1. Field of the Invention
The invention lies in the field of measurement technology and relates to a tracking method and to a measurement system with a laser tracker, according to the preambles of the respective patent claims. The tracking method serves for the automatic tracking of a target point, in particular of a moving target point, with the measurement beam of a laser tracker. The measurement system with the laser tracker is equipped for carrying out the method.
2. Description of Related Art
So-called laser trackers are frequently applied for measurement of the position of moving target points. The term laser trackers is to be understood as devices which comprise at least one distance meter operating with a focussed laser beam (called measurement beam in the following description). For example, the direction of the measurement beam is set to the target point with the help of a mirror which is rotatable about two axes, and is detected with angle sensors assigned to the rotation axes. The target point to be measured is provided with a retro-reflector (in particular cube-corner prism or arrangement of three mirrors which are perpendicular to one another), wherein the retroreflector reflects the measurement beam of the laser tracker which is incident thereon, back to this laser tracker. Thereby, the reflected measurement beam runs coaxially to the emitted measurement beam when the measurement beam hits the reflector in an exactly central manner, and runs offset parallel thereto, when the measurement beam does not hit the reflector in a centric manner. An absolute distance between the laser tracker and the target point and/or a change of this distance is deduced from a comparison of the emitted and reflected laser light, depending on the embodiment of the tracker. The position of the reflector or of the target point relative to the tracker is computed from the angles detected by the angle sensors and the distance detected by the distance meter.
A part of the reflected measurement beam is usually led onto a PSD (position sensitive device). One can infer the parallel shift of the reflected beam relative to the emitted measurement beam, from the position, in which the reflected measurement beam is incident on the light-sensitive surface of the PSD. The measurement data which is determined by way of this defines the parallel offset of the reflected measurement beam and is used for a control of the measurement beam direction, in a manner such that the measurement beam follows the target point (tracking) when this moves. This means that by way of a suitable change of the measurement beam direction or the alignment of the mirror aligning the measurement beam, one ensures that the parallel offset between the emitted and reflected measurement beam is reduced or remains as small as possible.
It is evident that the control of the measurement beam direction by way of the parallel offset between the emitted and the reflected measurement beam, although having a small delay, however has a delay which is not negligible and limits the speed at which a target point may move and thereby be tracked. If the target point moves more rapidly, the measurement beam, before its direction can be suitably corrected, no longer hits the reflector, and the tracking as well as positioning, are interrupted by way of this. The same may happen if an obstacle gets between the tracker and the target point, so that the measurement beam is interrupted. If the laser tracker or the measurement beam of the laser tracker “loses” the reflector, the operating person is made aware of this and a search routine can be started given a suitable design of the tracker.
The measurement of the position of the target point and its tracking by the measurement beam can be assumed again as soon as the target point is “found” again, which is to say that the measurement beam is again incident on the reflector and is reflected by this, for which the distance measurement must be newly initiated as the case may be. The mentioned tracking interruptions become more frequent, the less controlled are movements of the target point and the smaller are the applied reflector and the diameter of the measurement beam. The same conditions as during the mentioned tracking interruptions usually also prevail at the beginning of a measurement process, when the tracker is not at all yet set onto the target point.
It is also known to provide laser trackers with an overview apparatus. This camera which has an, as large as possible, field of view (for example over ±20° in all directions), is arranged on the tracker and is aligned in a manner such that the measurement beam can be directed onto a target point recognised on the camera picture. The alignment of the measurement beam onto this target point is initiated by an operating person observing the camera picture, by way of this operating person suitably indicating the picture region in which the target point is imaged.
A tracking method and a measurement system with a laser tracker which has two tracking modes, and the measurement system switches from one of the tracking modes into the other when the measurement beam of the laser tracker “loses” the target point or “finds it again”, is described in WO 2007/079601 A1. The normal or ordinary tracking mode is the tracking which is known for laser trackers and which is based on the measurement beam, in which thus for example, the parallel offset between the emitted and reflected measurement beam is detected and one strives for a reduction of this offset by way of changing the measurement beam direction. In the normal tracking mode, the tracker detects the measurement beam which is reflected by the reflector, and a determining of the target point position is possible at any time. In the extraordinary tracking mode, in which the measurement system operates when the tracker cannot detect the reflected measurement beam, the change of the measurement beam direction is controlled by way of data which is recorded by an overview apparatus assigned to the laser tracker. The overview apparatus, for example, is a digital overview camera which provides picture data and has a light-sensitive surface (e.g. CCD) and optics which give the overview camera a viewing angle for example of ±20° in all directions, which is common for an overview apparatus. The overview apparatus can however for example also be a PSD (position sensitive device) which is equipped with the same or similar optics and which only provides position data with respect to the sensor, thus direction data with regard to the apparatus. A direction to the reflector is determined from the data registered by the overview apparatus and, with a suitable change in the measurement beam direction, one attempts to direct this onto the reflector. The extraordinary tracking mode thus operates without a detection of the reflected measurement beam, and an exact determining of the position of the target point with the help of the tracker is not possible in the extraordinary tracking mode. The extraordinary tracking mode is switched on as soon as no reflected measurement beam is detected in the tracker. In the extraordinary tracking mode however, one always checks again and again, as to whether a reflected measurement beam is detected or not, and as soon as this is the case, the system switches again into the ordinary tracking mode and the position measurement is released.
The described device and the corresponding method thus can localise a “lost” target point again and thereafter again determine the position of the target point. However, their capability of following rapid changes in the angular position of the target point with regard to the tracker is however limited. This is of particular relevance, if the target point is located close to the tracker, and a given absolute position change—compared to a target point distanced further away—corresponds to a greater change of the angle at which the target point is seen from the tracker. Furthermore, it is necessary for the target point to be able to be held in a relatively calm manner for localising, until the capture is concluded and the position measurement can be activated again.
Similar tracking methods using cameras or sensors with a narrow or wide viewing angle are also described in the following three publications:
EP 2 071 283 A2 describes the use of two separate cameras with a wide and narrow viewing angle, in each case with their own light source coupled into the camera optics. The cameras are arranged separately from one another, one of these with the viewing axis colinear to a distance meter, and operate with visible light. A target recognition is accomplished in each case by switching on/off the respective light source and a subsequent difference formation from the respective pictures.
WO 2009/046763 A1 shows two stages with the target tracking, wherein one switches over between a close range setting of optics, with a wide viewing angle, and a long range setting with a narrow viewing angle.
U.S. Pat. No. 7,292,788 B2 describes a laser-based communication with a satellite, wherein a received light beam is tracked with wide field sensors and narrow field sensors. FIG. 4B shows an apparatus for a two-stage measurement: either an intermediate/acquisition track sensor (660) or a fine track quad cell (650) is applied, in order to lead a laser beam into a fibre-optic (640). Other embodiments or experimental arrangements (FIG. 4A) likewise use two-stage methods.